1. Field of the Invention
The present invention relates to an apparatus and method for synchronizing a transport packet in a ground wave digital multimedia broadcast and, more specifically, to an apparatus and method for synchronizing a transport packet in a ground wave digital multimedia broadcast wherein a normal decoding is performed in a receiver of a digital broadcasting system by synchronizing the transport packets in the receiver.
2. Description of the Related Art
Recently, as digital audio apparatus having excellent sound quality, such as compact disc (CD) and digital video disc (DVD) have increased in popularity, user demand for digital broadcasting with high quality sound has increased. Accordingly, in order to overcome the limitation of sound quality on an existing FM broadcast, digital audio broadcasting (DAB) has been implemented in European countries, Canada, United States or other countries. The DAB system provides an excellent receiving ability upon movement as well as a high quality sound using a technology different from existing AM or FM broadcasting and has a property of transmitting digital data such as image or text at a high speed. Recently, various multimedia services including image in addition to the audio broadcasting have been emphasized, which is referred to a digital multimedia broadcasting (DMB).
When the mobile terminal contains a DMB receiver or a DMB reception pack, it is possible to decode and display a moving picture encoded in the MPEG-4 standard in the mobile terminal. Accordingly, a user can be provided with various multimedia services through a mobile phone or a personal digital assistant (PDA). Here, the MPEG-4 was developed for the purpose of reception on movement to guarantee a reception of a good quality of program in fixed and mobile reception environments and to perform a role as media to provide a personal mobile broadcasting service since the program can be transferred through the mobile terminal (for example, on board unit, mobile phone and PDA).
In the MPEG-4 scheme, image signals are encoded on the basis of contents of the image, other than a conversion encoding scheme in a unit of block which is used in the H.261 standard, JPEG standard, and MPEG-1 and MPEG-2 standards. That is, in the MPEG-4 scheme, an image expression scheme based on the contents is employed and video objects each having attributes of screen shape information, movement information, and texture information are separated and processed. The image expression scheme based on the contents establishes an interrelation among the objects in a variety of multimedia applications and makes accesses and manipulations of them easy. That is, an object-oriented interactive function in the MPEG-4 deals with object elements of the screen and sound independently in the multimedia data access and couples them with one another using a link so that the user can freely construct the screen or sound. For example, while it was formerly possible to perform a process to change an actor's image while keeping the scenes on the screen, for example, only in the production step, the process can be performed in the user's step in the MPEG-4.
In the national standardization tasks for the DMB service, multimedia data in the sender is compressed and encoded in the MPEG-4 system taking consideration of the expandability of a variety of data services and transmitted to the MPEG-2 system together with meta-information. The MPEG-2 system packets the incoming MPEG-4 data in a packetized elementary stream (PES) format, makes it in a MPEG-2 transport stream (TS) format and transmits it using a Eureka system (the Eureka-147). Here, the Eureka system employs an orthogonal frequency division multiplexing (OFDM) transmission scheme together with time/frequency interleaving and error correction encoding in order to overcome a fading distortion caused by a transmission channel.
FIG. 1 is a view showing a construction of a DMB frame. Generally, a DMB frame transmission scheme includes a stream scheme and a packet scheme, and national ground wave DMB employs the stream scheme. Referring to FIG. 1, a symbol null is used to notify a start of a frame, and fast information channels (FICs) 10 have information on a sub-channel of common interleaved frames (CIFs) 20. Each CIF 20 includes maximum 64 sub-channels SCH0-SCH63 and each of the sub-channel SCH0-SCH63 includes an audio part (MPEG-1 Layer-II) and data part. The data part is divided into general data and a TS packet. The TS packet is a transmission format used to transmit the MPEG-2 stream, which includes the MPEG-4 stream in the TS packet in the case of the ground wave DMB.
FIG. 2 is a view showing a construction of a TS packet. Referring to FIG. 2, the length of the TS packet is 188 bytes, and the first 1 byte of the packet represents a starting point of the packet which starts with 0x47. Remaining 187 bytes are a portion where data is actually stored.
FIG. 3 is a block diagram showing an example of a transmitter of a general digital broadcasting system. Referring to FIG. 3, the transmitter of the digital broadcasting system generally includes a scrambler 310, a forward error correction (FEC) unit 330, and a modulator 340. The FEC unit 330 includes a Reed-Solomon (RS) encoder 312, an external interleaver 314, a converter 316, a convolution encoder 318, and an internal interleaver 320.
The scrambler 310 changes and randomizes each byte value of the TS packet of the incoming MPEG-2 format in a predetermined pattern.
The FEC unit 320 performs an encoding operation in order to correct errors that may occur while transmitting the TS packet data of 188 bytes input through the scrambler 310. The RS encoder (Reed-Solomon encoder) 312 receives the TS packet data output from the scrambler 310 and performs the RS encoding operation in a block to correct the error. A parity code to correct the error is added by the RS encoding operation. By doing so, the RS encoded TS packet data becomes 204 bytes. The external interleaver 314 rearranges the data encoded in the block in the RS encoder 312 and performs a function of distributing a burst error that may occur. The converter 316 converts the 204 byte TS packet data rearranged by the external interleaver 314 from bytes to bits. The convolution encoder 318 convolutionally encodes output the bits, and the convolutionally encoded bits are rearranged in the internal interleaver 320 and output. Accordingly, the channel encoded data is output.
The modulator 340 properly modulates the encoded data output from the FEC unit 330 according to a transmission scheme of a digital broadcasting system and transmits the DMB stream to the receiver.
The bits of the TS packet in the DMB stream are transmitted in a sub-channel, where a size of the sub-channel and a size of the TS packet are not synchronized. It is because while the size of the sub-channel is at least 64 bits, the size of the TS packet is 204*8 bits, so that a TS packet to sub-channel ratio equals about 25.5 which is not an integer when dividing 204*8 bits by 64 bits.
Accordingly, since the size of the sub-channel and the size of the TS packet are not synchronized, the starting points of the sub-channel and the starting point of the TS packet may not be coincidence with each other. That is, the TS packet can start in a middle portion of the sub-channel.
FIG. 4 is a block diagram showing an example of a receiver of a digital broadcasting system. Referring to FIG. 4, the receiver 400 of the digital broadcasting system includes a demodulator 410, an FEC unit 430 and a descrambler 440. The demodulator 410 demodulates the DMB stream received via an antenna. The FEC unit 430 corrects an error in a signal output from the demodulator 410, and decodes encoded data. The descrambler 440 descrambles data output from the FEC unit 430 and outputs the DMB stream including the TS packet.
Hereinafter, the FEC unit 430 will be described in a greater detail.
An internal deinterleaver 412 of the FEC unit 430 performs an internal deinterleaving operation corresponding to an interleaving operation performed by an internal interleaver of the sender. That is, the internal deinterleaver 412 performs an inverse operation of the internal interleaver of the sender. The signal deinterleaved by the internal deinterleaver 412 is transferred to the internal decoder 414, and then an internal decoding operation, corresponding to an encoding operation performed in the internal encoder of the sender is performed.
The signal decoded in the internal decoder 414 is converted from bits to bytes by the converter 416. The TS packet detector 418 detects the synchronized TS packet from the signal decoded by the byte. The synchronized TS packet is transferred to the external deinterleaver 420, where an external deinterleaving operation corresponding to an interleaving operation which is performed by the external interleaver of the sender is performed.
FIG. 5 is a view explaining a method for detecting a TS packet by a TS packet detector. Referring to FIG. 5, the TS packet detector 418 checks the TS packet byte by byte and then detects a synchronized byte ‘0x47’. When the synchronized byte ‘0x47’ is detected for the first time, the packet detector 418 checks whether the synchronized byte ‘0x47’ is detected every 188 bytes for three consecutive times. As a result, if the synchronized byte ‘0x47’ is consecutively detected three times, it is determined that the TS packet is detected.
In the case that the TS packet is initialized in the transmitter of the digital broadcasting system or contents of the TS packet are changed, a starting point of the TS packet existing in the sub-channel (SCH0-SCH63) of the CIF 20 may be changed.
In that case, the starting point of the TS packet is changed upon operation of the receiver, there is a problem that the receiver may lose a symbol synchronization by the byte due to a new starting point of the TS packet, and then the decoder cannot perform a normal decoding operation.